1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image. FIG. 1 is a sectional view of a fixing device 20R1 including a fixing belt 204. The fixing belt 204 is looped around a heating roller 202 and a fixing roller 203. A pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a fixing nip N between the pressing roller 205 and the fixing belt 204. The fixing belt 204 is heated by a heater 201 provided inside the heating roller 202. As a recording medium P bearing a toner image passes between the fixing roller 203 and the pressing roller 205 on the fixing belt 204, the fixing belt 204 and the pressing roller 205 apply heat and pressure to the recording medium P bearing the toner image to fix the toner image on the recording medium P.
One problem with such an arrangement, however, is that the heating roller 202 has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device 20R1. To address this problem, instead of the fixing belt 204 the fixing device may include a fixing film having a relatively small heat capacity. FIG. 2 is a sectional view of a fixing device 20R2 including a fixing film 213. A pressing roller 212 presses against a ceramic heater 211 provided inside a loop formed by the fixing film 213 via the fixing film 213 to form a fixing nip N between the pressing roller 212 and the fixing film 213. As a recording medium bearing a toner image passes between the pressing roller 212 and the fixing film 213, the fixing film 213 heated by the ceramic heater 211 and the pressing roller 212 apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.
However, the fixing film 213 also has a drawback in that, over time, friction between the ceramic heater 211 and the fixing film 213 sliding over the ceramic heater 211 increases, resulting eventually in unstable movement of the fixing film 213 and increasing the required driving torque of the fixing device 20R2. Further, the rotating fixing film 213 is heated by the ceramic heater 211 only locally, that is, at the fixing nip N, and therefore the fixing film 213 is at its lowest temperature just before entering the fixing nip N in a direction of rotation of the fixing film 213. Accordingly, when the fixing film 213 is rotated at high speed, the fixing film 213 passing through the fixing nip N may not have a proper fixing temperature, resulting in faulty image fixing.
To address the above-described problems, the fixing device may include a pipe-shaped metal member provided inside the fixing belt, with a slight gap provided between the fixing belt and the pipe-shaped metal member. A heater provided inside the pipe-shaped metal member heats the pipe-shaped metal member, which in turn heats the fixing belt, to maintain the fixing belt at the proper temperature.
Typically, in the case of fixing devices employing either a fixing belt or a fixing film, at the fixing nip a nip formation member is provided in a concavity formed in one side of the pipe-shaped metal member and pressed against the pressing roller via the fixing belt so as to provide a nip of sufficient length. The nip formation member is formed of rubber to absorb slight surface asperities of the recording medium passing through the fixing nip. However, when the rotating fixing belt slides over the stationary nip formation member formed of rubber, a substantial amount of friction may be generated between the fixing belt and the nip formation member.
To address this problem, a low-friction sheet may be used to cover the nip formation member to reduce the friction between the fixing belt and the nip formation member. However, the low-friction sheet needs to be attached to the nip formation member securely to resist a shearing force applied by the rotating fixing belt. For example, the low-friction sheet may be attached to the nip formation member with a plurality of screws. However, such an arrangement increases the number of component parts and thus complicates assembly of the fixing device, resulting in increased manufacturing and maintenance costs.